1. Field of the Invention
The disclosure relates to a reference voltage generation circuit to generate a reference voltage (i.e., an input variable voltage to which a higher limit value and a lower limit value are set) in response to receiving a variable voltage, and a power source device and a liquid crystal display device using the circuit.
2. Description of Related Art
FIG. 4 is a circuit diagram showing a first conventional example of a reference voltage generation circuit. A reference voltage generation circuit 70 of the first conventional example receives a temperature detection voltage VT (i.e., a voltage signal, the voltage value of which fluctuates according to temperature fluctuation) from a temperature sensor 60, and generates a reference voltage VREF by setting a higher limit voltage VH and a lower limit voltage VL to the temperature detection voltage VT (in reference to FIG. 3).
As a technique to realize the operation described above by using an analog signal, the reference voltage generation circuit 70 in accordance with the first conventional example includes a first amplifier circuit X which preferentially outputs a higher voltage between the temperature detection voltage VT and the lower limit voltage VL, and a second amplifier circuit Y which preferentially outputs a lower voltage as a reference voltage VREF between the output voltage VX provided from the first amplifier circuit X and the higher limit voltage VH.
As an input stage, the first amplifier circuit X is a construction which includes npn bipolar transistors X1 and X2, to each base terminal of which the temperature detection voltage VT and the lower limit voltage VL are inputted (i.e., the first amplifier circuit X is kind of a npn-input-type amplifier). As an input stage, the second amplifier circuit Y is a construction which includes pnp bipolar transistors Y1 and Y2, to each base terminal of which the output voltage VX and the higher limit voltage VH are inputted (i.e., the second amplifier circuit Y is kind of a pnp-input-type amplifier).
In addition, as an example of a technique related to the aforementioned conventional technique, Japanese patent publication No. 2009-232550 can be listed.
However, because the second amplifier circuit Y of pnp-input-type is used for the reference voltage generation circuit 70 of the first conventional example, at least a voltage value corresponding to the sum of the three voltage is required as a power source voltage to drive the input stage of the second amplifier circuit Y: the higher limit voltage VH applied to a base terminal of the transistor Y2, an ON threshold voltage Vf of the transistor Y2, and a drop voltage Vsat of the current source Y5 (i.e., VH+Vf+Vsat≈VH+1V). Therefore, with respect to the reference voltage generation circuit 70 of the first conventional example, a problem arises because a minimum operation voltage (i.e., a lowest value of the power source voltage required to maintain a normal operation) cannot be lowered adequately.
As shown in FIG. 5, as a technique to generate the reference voltage VREF (i.e., the temperature detection voltage VT to which the higher limit voltage VH and a lower limit voltage VL are set), a combination of buffers 91 to 93, comparators 94 to 95, a logic circuit 96 and a selector 97 which operates by digital signal, can be proposed. However, such a combination can result in an increase in circuit size or cost, as well as noise occurring during switching of the selector, deterioration of transient characteristics remains problems in addition to the aforementioned problems.